Calculate surface area of each material?
Extending the answer from surface area of a mesh using Python code in Blender 

import bpy
import bmesh

#active object
obj = bpy.context.active_object
# bmesh of active object
bm = bmesh.new()
bm.from_mesh(obj.data)
#material slots
for i, slot in enumerate(obj.material_slots):     
    area = sum(f.calc_area() for f in bm.faces
               if f.material_index == i)
    print("Material %d Area %.4f" % (i, area))

bm.free()